Don't throw an exception when R not installed (fixes #48)

With this patch, when R is not installed we get this:

![image](https://f.cloud.github.com/assets/738761/1414028/c067c846-3e64-11e3-9990-28def4ac4dd5.png)

and this:

![image](https://f.cloud.github.com/assets/738761/1414027/a9dbd2ca-3e64-11e3-916d-00e7a6aff496.png)